When I replaced my falling down shed last autumn I decided I would make room for a polytunnel. My thinking was that it would complement the perennial planting in the rest of the garden and give me space to grow some annual vegetables. I spent a while researching and planning it out before settling on one from First Tunnels. I've been really happy with it and would recommend them.

Because I finished construction in the early autumn I put in just a few plants that would get going before the winter. I used this book as a guide: How To Grow Food In Your Polytunnel. I grew some salad - lettuce, rocket and mizuna; onions and spring onions; cabbage, kohlrabi and peas. I also had some herbs that I brought in from the garden in containers - parsley, mint, chives and thyme. I also grew some coriander from seed. I also tried growing some potatoes in bags for harvest in December.

The salads have been fantastic. Once established they provided plenty of leaves right through the coldest months. I draped fleece over them when there was frequent frost and that seemed to be enough protection to stop all but a few frost-burnt tips. The onions, cabbage and kohlrabi are growing nicely. The peas have done well, but I have tried growing them up string and think they would have done better up solid sticks. The potatoes weren't a success. I think I left them a bit late to put in and by the time I harvested them at Christmas there were hardly any tubers. On the whole I was very happy with the outcome.

Now that the days are getting longer and temperatures rising I've been getting the tunnel ready for the spring and summer. I've been improving the soil in the rest of the tunnel that I haven't planted so far. I've been using compost from our local tip. It's only £1 for a 40 L tub if you fill it yourself and is very rich. I'm planning on growing tomatoes, chilli peppers, cucumbers and courgettes amongst other things so I've been adding compost to the thin chalky soil. Now that I've mixed it in and watered it I'm planning to leave it alone until the seedlings are ready to go in. The tomato and chilli seeds are planted out in modules now and the others will follow soon. I'm also sowing more salads to replace the overwintering ones.

To make a little more use of the space I've put in a couple of lengths of square section guttering on the south side of the tunnel above the benches where I raise the seeds. I've filled them with soil and have put in some strawberry plants. The higher temperatures higher in the tunnel should bring them on more quickly. They will need care to make sure they don't dry out though.

South side of polytunnel with strawberries in guttering suspended from crop bars above staging, with peas, kohlrabi, lettuce, cabbage, onions, rocket and mizuna below.

Up until now I've been keeping the doors shut for most of the day, but I am just starting to leave one open in the day time to increase the ventilation. Even during the winter the temperature can reach high 30's - low 40's (degrees C) during sunny spells. Once the other crops are in there I'm guessing I will have to leave the doors open every day.

I've been measuring the temperature in the polytunnel and seeing how it compares to that outside. I had also read about how a water butt in the polytunnel can help moderate the temperature, especially overnight, so I put another temperature sensor in the water. The image below shows how the temperatures have changed over the last few months. Date is left to right and hour is top to bottom.

Average hourly temperatures outdoors, in the polytunnel and in the polytunnel water butt from the start of 2015.

The top plot shows that the outdoor temperature doesn't get much above about 12 deg C for most of the time. It is slightly warmer in the middle of the day, but it is about the same as a warm weather front coming through for a few days.

The air temperature in the polytunnel (the middle plot) is much warmer in the middle of the day when the sun is shining. The warmer hours clearly get longer over this period and the temperatures rise. However, for the rest of the day the temperatures are similar to the outdoor ones. Having said that, I think the plants benefit from being protected from the wind during those cold hours and the added benefit of fleece for some of them. The warmer temperatures in the middle of the day are definitely warm enough to allow plants to grow.

The water butt temperature in the lower plot is interesting. It shows some of the variation in temperature that the outdoors sees but it is generally warmer. The temperature rises after midday and I believe this is due to the sun shining on the surface of the water butt. The temperature remains high through to the early morning. The water temperature seems to correlate with the high midday air temperatures in the polytunnel caused by the sun. I'm not sure how much the water butt helps the air temperatures in the polytunnel as whole. The air temperature sensor is on the other side of the tunnel about 1 m off the ground. However, I suspect is will help the plants close to it. Also, the paving path along the centre is also probably acting to absorb the heat and release it more slowly, helping out the plants close to the ground.

At the moment I don't have a temperature sensor in the soil. I'm thinking it might be a good idea to help time planting things out.

Your graphs are interesting. You would expect that the water butt would keep the poly tunnel warmer during the night as stored heat is released but what is odd is that your poly tunnel night time temps are lower than outdoors even though the water butt is warmer than out doors. Probably the water butt is too small to have much of an impact but that still leaves the odd night time temps inside being colder than outdoors.

I guess that your out door thermometer is higher up than the indoor one (sorry if I missed that in your blog), the ground / earth in the poly tunnel is probably cooling the air as it is moist.

I think to see the effect of the water butt you would need several sensors placed around the water butt at different heights.

I often see air temps at 10ft up outside being 3 to 5 degrees warmer than the ground during the night / early morning.

I f the poly tunnel thermometer is 1m up then the outdoor therm should also be 1m up I think.

If you look at my website link, to my insolation meter, you can match the power in the sun to your poly tunnel temps. A little bit of maths will show you how much power is being put into the poly tunnel and warming the soil and also how long it takes to release that heat back into the air of the poly tunnel thereby keeping the tunnel warmer during the night. A very quick look suggests the soil (maybe also the water butt) is releasing heat for a couple of hours.

It can give you some idea of how much soil or water in a butt (a thermal mass) you need to extend the warmth of your tunnel longer into the night.

thanks for your comments. Well spotted! Yes, my outdoor temperature monitor is at about 3 m above ground. Conveniently placed outside a window in the house rather than specifically for comparison with the poytunnel. Not ideal obviously, but enough to capture the variation in outdoor temperature.

Good idea about working out the insolation. I've been thinking of doing some sums to work out how much energy is being captured by the water butt. I've got solar panels on the house now so can get 15 min solar energy data, although it is a little bit more manual than the automated Raspberry Pi set up for temperature monitoring. It would be interesting to work out how much it would warm the air by if it gave up a fraction of that heat overnight.

My temperature monitor in the polytunnel is only about 1 m away from the water butt. I think you are right, the water butt probably does warm the air very close by. It doesn't look as though it makes a large difference to air temperature at 1 m away, but I wonder if the radiated heat might help the plants stay a little warmer too.

I think I ought to get a soil temperature monitor set up too so that I can see how that changes too.

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A blog following the development of a small edible forest garden in Salisbury, Wiltshire, first planted in winter 2008.